In gas analyzers which measure the oxides of nitrogen content of a sample gas stream, it is the practice to perform the measurement in two stages. In the first stage, the sample gas stream is mixed with an ozone containing gas stream and the photons emitted as a result of the reaction therebetween are measured with a photomultiplier tube to obtain a first oxides of nitrogen reading. In the second stage, the higher oxides of nitrogen in the sample gas stream, known collectively as NOx, are reduced to nitric oxide by the action of a catalytic converter that is commonly referred to as an NOx converter. After reduction, the gas stream is again mixed with an ozone containing gas stream, the resulting photons being measured with a photomultiplier tube to obtain a second oxides of nitrogen reading. The first reading is then taken as the NO content of the sample stream, and the difference between the second and first readings is taken as the NOx content of the sample stream.
Because the efficiency of the NOx converter in reducing higher oxides of nitrogen to nitric oxide is crucial to the accuracy of analyzers of the abovedescribed type, it has become the practice to include measurements of the efficiency of the NOx converter as a regular part of the procedure for using oxides of nitrogen analyzers. The method by which the efficiency of the NOx converter is measured involves applying to the sample inlet of the analyzer a series of gases the compositions and flow rates of which are related to the operating conditions of the analyzer. At various points in the measurement process analyzer output readings are recorded for later substitution into an equation, the solution of which yields the desired NOx conversion efficiency.
Both the above conversion efficiency equation and the readings to be recorded for substitution therein are well known to those skilled in the art. Moreover, the conversion efficiency equation, the readings used therein, and the procedures and conditions under which the readings are taken form the subject matter of regulations which have been promulgated by the Environmental Protection Agency for purposes of standardization. The latter regulations are set forth in Title 40 of the Code of Federal Regulations, Section 86.132-84, entitled "Oxides of Nitrogen Analyzer Calibration". The text of these regulations is hereby expressly incorporated herein by reference.
In order to facilitate the practice of the subject method, there have been developed special devices, known as NOx converter efficiency detectors, that are designed to aid in establishing and adjusting the gas flows used during the measurement process. In a typical one of such detectors, there was provided an inlet for oxygen or other oxygen containing gas, a second inlet for supplying a gas of known nitric oxide content, and an outlet for connection to the sample inlet of an oxides of nitrogen analyzer. Also included in such converter efficiency detectors was an ozone generator for converting a portion of the oxygen in the oxygen containing gas into ozone. The desired ozone concentration was then established by varying the operating voltage applied to the ozone generator. Finally, the efficiency detectors included various valves and pressure regulators by means of which the flow of gases could be initiated and adjusted.
While NOx converter efficiency detectors of the above-described type greatly facilitated the efficiency measurement process, they had several shortcomings which limited their utility. One shortcoming was the relative difficulty of making the final, fine adjustments of the ozone level in the oxygen containing gas stream. Small changes in the excitation of the ozone generator could, for example, cause relatively large changes in ozone concentration. This, in turn, led to an iterative adjustment process that greatly increased the amount of time necessary to complete the overall efficiency measurement.
A second shortcoming of converter efficiency detectors of the above-described type is that they are subject to unanticipated fluctuations in the flow rates of the gases that are applied to the inlet of the oxides of nitrogen analyzer. If, for example, the quantity of the oxygen containing gas that was added to the nitric oxide containing gas was changed, changes would occur in the rate of flow of the mixture of these gases at the input of the analyzer. As a result, the accuracy of the efficiency computation, which is based on an assumed constant overall gas flow rate, was adversely affected. Thus, the interdependence of the gas flow rates within the detector tended to reduce the overall accuracy of converter efficiency measurements.